Systems and methods reduce temperature induced drift effects on a liquid lens

ABSTRACT

Systems and methods reduce temperature induced drift effects on a liquid lens used in a vision system. A feedback loop receives a temperature value from a temperature sensor, and based on the received temperature value, controls a power to the heating element based on a difference between the measured temperature of the liquid lens and a predetermined control temperature to maintain the temperature value within a predetermined control temperature range to reduce the effects of drift. A processor can also control a bias signal applied to the lens or a lens actuator to control temperature variations and the associated induced drift effects. An image sharpness can also be determined over a series of images, alone or in combination with controlling the temperature of the liquid lens, to adjust a focal distance of the lens.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of pending U.S. patent application Ser.No. 14/145,185, filed Dec. 31, 2013, and entitled “SYSTEMS AND METHODSREDUCE TEMPERATURE INDUCED DRIFT EFFECTS ON A LIQUID LENS,” the entirecontents of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE TECHNOLOGY

The present technology relates to adjustable lenses used in a lenssystem, and more specifically, to systems and methods for reducingtemperature induced drift effects on a micro-fluidic or liquid lens usedin a vision system.

Vision systems have been developed for many different applications. Forexample, machine vision systems have been developed for reading barcodes and other types of symbols placed on packages or products toobtain information there from. Other machine vision systems have beendeveloped for inspecting manufactured parts forfeatures/characteristics.

Many vision systems include a camera for obtaining images of symbols oritems to be imaged. A processor receives the images and extractsinformation that can then be used to perform one or more visionprocesses. In many applications, the distance between a camera sensorand a symbol or item to be imaged can vary between uses. In these cases,in order to obtain useful images, i.e., images from which data requiredto complete machine vision processes can be extracted, an adjustablelens and/or auto-focus system is often provided. In these cases, whenthe system is activated to perform a vision process, the lens andauto-focus system automatically focus the lens so that a clear image ofthe symbol or item to be imaged is generated on the camera sensor. Afterthe focusing process is completed, a clear image of the symbol or itemto be imaged is obtained and is processed to complete the visionprocess.

One type of adjustable lens that can be used in a machine vision systemis a liquid lens. Liquid lenses are constructed of one or more fluids ofdifferent refractive indexes, and can be varied by controlling themeniscus, or surface of the liquid. In one type of liquid lens, forexample, two fluids are contained in a tube with transparent end caps.The first is an electrically conducting aqueous solution, and the secondis a non-conducting oil. The interior of the tube is coated with ahydrophobic material, which causes the aqueous solution to form ahemispherical lens which can be adjusted by applying a DC voltage acrossthe coating to decrease its water repellency in a process calledelectrowetting. Electrowetting adjusts the surface tension of theliquid, which changes the radius of curvature and adjusts the focallength of the lens. Several liquid lens configurations utilizing anelectrowetting process are known.

Another type of adjustable liquid lens utilizes an electrical/mechanicalactuator system to induce movement to adjust the focus of the lens. Forexample, a voice coil type adjustable lens has a ring shaped voice coilactuator that presses onto a transparent membrane serving as atransparent sidewall of a container. The container is filled with atransparent liquid. A current applied through the actuator induces theactuator to apply a force to deform the membrane into a convex shape.The convex shape acts as the lens, and can be adjusted by adjusting thecurrent.

Liquid lenses are extremely versatile, providing a highly variable focallength, and some without the need for moving parts. Liquid lenses,however, are inherently subject to undesirable changes in focal length(referred to herein as drift) due to temperature changes and aging ofthe liquids in the lens. Temperature and aging can, for example, alterthe refractive index of the liquids, or the dielectric constant, therebychanging the focal length. For example, when small symbols are imaged ata fixed large distance, a temperature drift of the lens will cause blurin the image and decrease reading performance. This undesirable driftcauses the liquid lens at a first temperature to have a first focallength, and the same liquid lens at a second temperature would have asecond focal length different from the first focal length.

For adjustable lenses that use a current applied through the actuator toadjust the focus of the lens, the current applied through the actuatornot only heats the actuator, but the lens heats up as well. Undesirably,this causes the temperature of the lens to vary with the applied controlcurrent. At large optical power (close object distances) the lens willheat up more than when used at small optical power (large objectdistance) due to the higher current need for the larger optical power.

Attempts have been made to compensate for liquid lens drift. Theseattempts measure the thermal behavior of the liquid lens during acalibration process, and then compensate the lens at normal operationbased on the measured thermal behavior by adjusting the liquid lensdriver voltage or current. This not only requires a time consumingcalibration process for each lens, but the measured thermal behavior ismade based on a typical drift behavior during calibration, which haslimited accuracy.

Therefore, when using a variable lens in applications that inducechanges in the temperature of the lens, the focusing of the variablelens will produce different results at different temperatures. For theseapplications, other systems and methods must be used in an attempt tomaintain a more consistent focal length and a sharper resulting image.The present technology addresses solutions to these issues.

BRIEF SUMMARY OF THE TECHNOLOGY

The present technology provides systems and methods for reducingtemperature induced drift effects on a liquid lens used in a visionsystem. A processor can receive a temperature value from a temperaturesensor, and based on the received temperature value, energize orde-energize a heating element on at least one circuit board to maintainthe temperature value within a predetermined control temperature rangeto reduce the effects of drift. The processor can also control a biassignal applied to the lens or a lens actuator to control temperaturevariations and the associated induced drift effects. An image sharpnesscan also be determined over a series of images, alone or in combinationwith controlling the temperature of the liquid lens, to adjust a focaldistance of the lens.

In one aspect, the present technology provides vision systems andmethods for maintaining the temperature of the liquid lens at a controltemperature, thereby reducing drift effects on the liquid lens. Thevision system includes an adjustable focus liquid lens having a field ofview. At least one circuit board is in thermal contact with at least aportion of the liquid lens. A heating element is positioned on the atleast one circuit board, the heating element controllable to heat the atleast one circuit board. A temperature sensor is positioned to measure atemperature value of the liquid lens. A feedback loop controls a powerto the heating element based on a difference between the measuredtemperature of the liquid lens and a predetermined control temperature.

In other aspects, the present technology provides vision systems andmethods for controlling a bias signal to the liquid lens to control thetemperature of the liquid lens. The vision system includes an adjustablefocus liquid lens having a field of view, the focus of the liquid lensbeing adjustable with a control signal applied to the liquid lens forcapture of an image. A bias signal is applied to the liquid lens whenthe liquid lens is not adjusted with the control signal for capture ofthe image. The bias signal being applied to the liquid lens to control atemperature of the liquid lens.

In some embodiments, the bias signal can be controlled in relation to anaverage dissipation of heat from the liquid lens. In other embodiments,the bias signal can be dependent on a sensed temperature value of theliquid lens or ambient temperature.

Other embodiments comprise systems and methods that optimize the focaldistance of an adjustable lens in a vision system, the vision systemhaving a field of view. The method comprises several steps includingadjusting the focal distance of the adjustable lens by a predeterminedadjustment step; acquiring a first image of the field of view thatincludes a region of interest; calculating a first sharpness score forthe region of interest that is within the first image of the field ofview; adjusting the focal distance of the adjustable lens by thepredetermined adjustment step; acquiring another image of the field ofview that includes the region of interest; calculating another sharpnessscore for the region of interest that is within the another image of thefield of view; comparing the first sharpness score with the anothersharpness score; and defining a direction of a next adjustment step inthe focus distance based on the comparison.

Yet other embodiments comprise systems and methods that optimize thefocal distance of an adjustable lens in a vision system, the visionsystem having a field of view. The method comprises several stepsincluding adjusting the focal distance of the adjustable lens by apredetermined adjustment step; acquiring a first image of the field ofview; measuring a first ambient temperature near the adjustable lens;adjusting the focal distance of the adjustable lens by the predeterminedadjustment step; acquiring another image of the field of view; measuringanother ambient temperature near the adjustable lens; comparing thefirst ambient temperature with the another ambient temperature; anddefining a direction of a next adjustment step in the focus distancebased on the comparison.

To the accomplishment of the foregoing and related ends, the technology,then, comprises the features hereinafter fully described. The followingdescription and the annexed drawings set forth in detail certainillustrative aspects of the technology. However, these aspects areindicative of but a few of the various ways in which the principles ofthe technology can be employed. Other aspects, advantages and novelfeatures of the technology will become apparent from the followingdetailed description of the technology when considered in conjunctionwith the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of a fixed-mount reader device obtaining animage of a symbol on an item of interest according to embodiments of thetechnology;

FIG. 2 is a perspective view of the fixed-mount reader deviceillustrating a front end of the reader device;

FIG. 3 is a schematic diagram illustrating components that can comprisethe reader device of FIGS. 1 and 2;

FIG. 4 is an exploded view illustrating an embodiment of a liquid lensand components of the reader device that are positioned in a thermalrelationship to the liquid lens;

FIG. 5 is a schematic diagram illustrating values and data storable inmemory;

FIG. 6 is a side schematic view illustrating the liquid lens and circuitboards in contact with the liquid lens;

FIG. 7 is a flow chart of a method associated with controlling thetemperature of the liquid lens;

FIG. 8 is a side schematic view illustrating an additional embodiment ofa liquid lens including an actuator, and circuit boards in contact withthe liquid lens;

FIG. 9 is a chart showing relative positions a liquid lens is driven to,and the associated default positions the lens is returned to;

FIG. 10 is a chart similar to FIG. 9 and showing the same relativepositions the liquid lens is driven to, and instead showing calculatedreturn positions the lens is returned to for controlling the temperatureof the liquid lens; and

FIGS. 11, 12, and 13 are flow charts of methods associated withcontrolling the temperature of the liquid lens according to embodimentsof the technology.

While the technology is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the technology to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the technology as defined by the appended claims.

DETAILED DESCRIPTION OF THE TECHNOLOGY

The various aspects of the subject technology are now described withreference to the annexed drawings, wherein like reference numeralscorrespond to similar elements throughout the several views. It shouldbe understood, however, that the drawings and detailed descriptionhereafter relating thereto are not intended to limit the claimed subjectmatter to the particular form disclosed. Rather, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the claimed subject matter.

As used herein, the terms “component,” “system,” “method” and the likeare intended to refer to either hardware, a combination of hardware andsoftware, software, or software in execution. The word “exemplary” isused herein to mean serving as an example, instance, or illustration.Any aspect or design described herein as “exemplary” is not necessarilyto be construed as preferred or advantageous over other aspects ordesigns.

Furthermore, the disclosed subject matter may be implemented as asystem, method, apparatus, or article of manufacture using standardprogramming and/or engineering techniques and/or programming to producehardware, firmware, software, or any combination thereof to implementaspects detailed herein.

Unless specified or limited otherwise, the terms “connected,” and“coupled” and variations thereof are used broadly and encompass bothdirect and indirect mountings, connections, supports, and couplings.Further, “connected” and “coupled” are not restricted to physical ormechanical connections or couplings. As used herein, unless expresslystated otherwise, “connected” means that one element/feature is directlyor indirectly connected to another element/feature, and not necessarilyelectrically or mechanically. Likewise, unless expressly statedotherwise, “coupled” means that one element/feature is directly orindirectly coupled to another element/feature, and not necessarilyelectrically or mechanically.

As used herein, the term “processor” may include one or more processorsand memories and/or one or more programmable hardware elements. As usedherein, the term “processor” is intended to include any of types ofprocessors, CPUs, microcontrollers, digital signal processors, or otherdevices capable of executing software instructions.

As used herein, the term “memory” includes a non-volatile medium, e.g.,a magnetic media or hard disk, optical storage, or flash memory; avolatile medium, such as system memory, e.g., random access memory (RAM)such as DRAM, SRAM, EDO RAM, RAMBUS RAM, DR DRAM, etc.; or aninstallation medium, such as software media, e.g., a CD-ROM, on whichconfiguration data and programs may be stored and/or data communicationsmay be buffered. The term “memory” may also include other types of knownor future developed memory or combinations thereof.

Embodiments of the technology are described below by using diagrams toillustrate either the structure or processing of embodiments used toimplement the present technology. Using the diagrams in this manner topresent embodiments of the technology should not be construed aslimiting of its scope. The present technology contemplates systems andmethods for reducing and/or controlling temperature induced drifteffects on an adjustable lens, and improving image quality.

The various embodiments will be described in connection with a liquidlens as part of a fixed-mount symbol reader, the reader adapted toacquire an image of an object and/or a mark on the object. That isbecause the features and advantages of the technology are well suitedfor this purpose. Still, it should be appreciated that the variousaspects of the technology can be applied in other forms of electronicdevices and is not limited to use of a liquid lens as part of a reader,as it will be understood that a wide variety of electronic devices thatincorporate a heat sensitive lens may benefit from reducing temperatureinduced drift according to the features described herein.

Referring now to the drawings wherein like reference numerals correspondwith similar elements throughout the several views and, morespecifically, referring to FIG. 1, the present technology will bedescribed in the context of an exemplary fixed mount symbol reader 20that can be used to obtain images of symbols, e.g., two dimensionalsymbol 22, placed on a surface of an item 24 and that can decode thesymbols in the obtained images. While the technologies herein aredescribed in the context of a fixed-mount symbol reader 20, for examplewhere a conveyor moves items or packages of various sizes through thefield of view of the reader 20 such that the distance between the readerlens/sensor and the surface of a package or item on which the symbol isapplied may vary item to item, it should be appreciated that thetechnologies may also be useful in hand-held symbol readers as well asstationary cameras, as non-limiting examples.

Referring now to FIGS. 1 and 2, reader 20 can include a metal or rigidplastic housing 26. An adjustable focal length lens 36 can be providedbehind a lens housing 40 positioned near the distal end of the readerhousing 26, and has a field of view 42. Lens 36 can be a knownmulti-focal liquid lens that is commercially available. In these typesof lenses, the focal length is adjusted by varying a control signalapplied to the liquid lens.

Referring now to FIG. 3, in addition to the components described abovewith respect to FIGS. 1 and 2, reader 20 can include a processor 50, acamera sensor 52, a power source 54, memory 56, and one or moreinterface devices 58, such as an audible sound generator, an LED forindicating successful symbol decoding, wireless and/or wiredcommunications, etc. As would be known, the power source 54 could bereplaced with a battery to provide power. Processor 50 can be coupled tomemory 56 where programs performed by processor 50 can be stored. Inaddition, processor 50 can direct the storage of images obtained viacamera sensor 52 in the memory 56. Processor 50 can also be coupled tocamera sensor 52 for receiving image data there from. Knowntrigger/actuator devices or methods 34 can be coupled to or performed byprocessor 50 for initiating a symbol reading process. Processor 50 canalso be coupled to the variable focus liquid lens 36 for modifying thefocus position or focal length of the liquid lens 36.

In typical operation, the reader 20 is positioned such that the cameraor lens field of view 42 is directed toward a surface of the item 24 onwhich the symbol 22 has been applied so that the symbol 22 is disposedwithin the reader's field of view 42. Once so positioned, the trigger 34can be activated causing reader 20 to obtain one or more images of thesymbol 22 within the field of view 42. Once a suitably focused image ofsymbol 22 has been obtained, the processor 50 within reader 20, or usingthe communication interface 58, a processor remote from the reader 20,can attempt to decode the symbol 22 and can then provide the decodedinformation to other software applications for use. In addition, aftersuccessful decoding of the symbol 22, reader 20 may provide anindication to the user that decoding has been successful. Here, althoughnot illustrated in FIG. 1 or 2, the indication of successful decodingmay be provided via an audible beep or noise or via illumination of anLED or the like, or both.

Liquid lenses, such as liquid lens 36, are typically constructed of oneor more fluids of different refractive indexes, and can be varied bycontrolling the meniscus, or surface of the liquid. Liquid lenses can beadjusted by application of a control signal 64 to the liquid lens or toa liquid lens actuator. The control signal 64 can comprise a controlvoltage or a control current, for example. In some types of known liquidlens, for example, two fluids are contained in a tube with transparentend caps. The first is an electrically conducting aqueous solution, andthe second is a non-conducting oil. The interior of the tube is coatedwith a hydrophobic material, which causes the aqueous solution to form ahemispherical lens that can be adjusted by applying a DC voltage acrossthe coating to decrease its water repellency in a process calledelectrowetting. Electrowetting adjusts the surface tension of the liquidchanging the radius of curvature and adjusting the focal length of theliquid lens.

As discussed above, the optical properties of liquid lenses differ fromthose of typical glass or plastic lenses. The optical power of a liquidlens, for example, decreases as the temperature of the lens increases,and as the lens ages. When focusing the liquid lens, moreover, there ishysteresis between the control signal 64 and the optical power. That is,as the control signal 64 is increased and decreased, the incrementalchange in optical power varies, which can detrimentally affect feedbackloops.

Embodiments of the technology control the temperature of the adjustablelens 36 so as to reduce the drift effects caused by changes in the lenstemperature. To minimize the drift effects, the application of heat canbe controlled alone or in combination with controlling aspects of a biassignal 66 to the lens 36 or a lens actuator 96. As described below, thecontrol signal 64 can be removed between the acquisition of consecutiveimages. The bias signal 66 can be applied in place of the control signal64. The bias signal 66 can comprise a bias voltage or a bias current,for example. Adjustments can be made in the level of the bias signal 66and the length of time the bias signal is applied. When adjustments aremade in this way, the effects of temperature, both ambient temperatureand lens temperature, can be counteracted.

Generally, higher temperatures cause the optical power of the liquidlens 36 to decrease. In this example, current methods increase the focaldistance of the reader 20 to adjust for the decrease in optical power. Achange in focal distance can be used to compensate for the effect oftemperature on the liquid lens, but any time the liquid lens focus ischanged, there is risk associated with reducing the sharpness of theimages acquired due to the uncertainty of the exact focus the liquidlens should be adjusted to.

Referring now to FIG. 4, an embodiment is shown that can be used tosignificantly reduce or eliminate the focal drift in the liquid lens 36by stabilizing the temperature of the liquid lens 36. In thisembodiment, a portion of the housing 26 has been removed to provide anexploded view of the liquid lens 36 and components that are positionedin contact with and/or near the liquid lens 36. In this embodiment, theliquid lens 36 can be kept at a predetermined control temperature 60while variations of an ambient temperature 62 can occur surrounding thereader 20. Data such as the predetermined control temperature values 61and ambient temperature values 63 can be stored in memory 56 (see FIG.5). The ambient temperature 62 can be measured at or near the liquidlens 36 within the housing 26, or the ambient temperature 62 can bemeasured outside of the reader 30, or both. The control temperature 60can be maintained at a constant temperature and/or the controltemperature can be maintained at a near constant temperature, e.g.,within a range of several degrees. Further, the control temperature 60can be maintained to be within an operating range of the liquid lens 36,e.g., minus 50 degrees Celsius to 70 degrees Celsius.

In some embodiments, the control temperature 60 can be maintained at ornear the high end of the operating range, e.g., 70 degrees Celsius. Someliquid lenses change to a new focal distance quicker at highertemperatures. Therefore, maintaining the control temperature 60 at ornear the high end of the operating range would not only provide anoperating range of the reader 20 to be as large as possible, but wouldalso serve to reduce or eliminate the drift and improve the focusingspeed of the liquid lens 36 due to improved reaction time of the liquidsin the liquid lens. It is contemplated that the control temperature 60can be maintained at a low, or mid-range temperature, or any temperaturewithin the operating range that is at or above the ambient temperature,for example.

Referring now to FIGS. 4, 5 and 6, and by way of a non-limiting example,the liquid lens 36 can be positioned in thermal and/or physical contactwith a first circuit board 70 or between, e.g., thermal and/or physicalcontact, the first circuit board 70 and a second circuit board 72. Oneor both of the first circuit board 70 and the second circuit board 72can include a temperature sensor 74 as part of the control circuitry 76for the liquid lens 36 and/or the reader 20. By way of example, thefirst circuit board 70 can include contacts 78 to electrically couplethe control circuitry 76 to the liquid lens 36, and the controlcircuitry 76 on the second circuit board 72 can include liquid lensdriver circuitry. A control cable 80 can extend from the second circuitboard 72 to electrically connect the control circuitry 76 to theprocessor 50. A rubber ring 88 can be included to keep a constantpressure on one or both of the first circuit board 70 and a secondcircuit board 72 with the liquid lens 36 in-between. It is to beappreciated that other configurations and arrangement of components arecontemplated.

In some embodiments, one or both of the first circuit board 70 and thesecond circuit board 72 can be made from a thermally conductivematerial. An exemplary thermally conductive material is Thermal CladInsulated Metal Substrate developed by The Bergquist Company. Further,one or both of the first circuit board 70 and the second circuit board72 can include a controllable heating element 82. The heating element 82can be controlled to heat the circuit board it is on, e.g., the secondcircuit board 72, and to heat the ambient air at or near the liquid lens36.

In some embodiments, one or both of the first circuit board 70 and thesecond circuit board 72 can be in electrical, thermal and/or physicalcontact with the liquid lens 36. When in thermal contact, or physicalcontact, the heating element 82 can be controlled to generate a heatthat thermally affects the liquid lens 36. Referring to FIG. 7, a method83 is shown for controlling the temperature of the liquid lens. Atprocess block 84, the temperature sensor 74 can sense a temperaturevalue 132 associated with the liquid lens 36. At decision block 85, afeedback loop can compare the temperature value 132 to the controltemperature 60. If the temperature value 132 is not at the controltemperature 60 or within the control temperature range, at process block86, the heating element 82 can be energized to increase the temperatureof one or both of the first circuit board 70 and the second circuitboard 72, and in turn, the temperature of the liquid lens 36. At processblock 87, when the temperature value 132 is at the control temperature60 or within the control temperature range, the heating element 82 canbe de-energized, and the liquid lens properties can be maintained.

Additional reader 20 components, when assembled, can enclose the liquidlens 36 and the first circuit board 70 and the second circuit board 72.For example, a guide 90 and the lens housing 40 can physically andthermally enclose all or a portion of the liquid lens 36. Lens barrel 94and the lens housing 40 can physically and thermally enclose all or aportion of the liquid lens 36 and the first circuit board 70 and thesecond circuit board 72. Guide 90 can serve to center the liquid lens 36within the lens barrel 94. Any of the additional components, e.g., therubber ring 88, the guide 90, the lens housing 40, and the lens barrel94 can be further optimized for thermal insulation, e.g., by adjustingshape and material properties, in such way that only a minimum of powerwill be needed to keep the liquid lens 36 at the control temperature 60.

In an additional embodiment, the focal drift in the liquid lens 36 canbe reduced or eliminated by stabilizing the temperature of the liquidlens 36. This embodiment can be used alone, or in combination withembodiments described above and shown in FIGS. 4 to 7.

For example, other known adjustable lens configurations utilizeelectrical/mechanical actuator systems such as piezoelectric actuators,small motors, and electromagnetic actuators, e.g., a voice coil, toinduce movement to control a lens or lenses, e.g., the meniscus of aliquid lens. In some embodiments, other variable lens elements are alsoused, for example, by changing the refractive index of a transparentmaterial. FIG. 8 shows an exemplary variable lens 95. The variable lens95 can include a ring shaped voice coil actuator 96 that is induced topress onto a transparent membrane 98 serving as a transparent sidewallof a container 108. The container is filled with liquid 36. A controlsignal 64 applied through the voice coil 99 induces the actuator 98 toapply a force to deform the membrane 98 into a convex shape. The convexshape acts as the liquid lens 36, and can be adjusted by adjusting thecontrol signal 64. In these liquid lens configurations, the actuator 96itself can induce temperature variations of the liquid lens 36 due tothe control signal 64 applied to the actuator to change the focus of theliquid lens. The power dissipation in the actuator 96 is generallyproportional to the square power of the control signal 64. For example,when the liquid lens 36 is driven to provide a high optical power, e.g.,to focus in on a close symbol, more control current to the actuator 96is required and the heat generation and associated dissipation from theliquid lens 36 is high. Conversely, when the liquid lens 36 is driven ata lower optical power, e.g., to focus in on a farther symbol, lesscontrol current to the actuator is required and the heat generation andassociated dissipation from the liquid lens 36 is lower. In someapplications, the induced temperature variations in the liquid lens 36can be a challenge to accurately detect with the temperature sensor 74,as the thermal coupling between the actuator 96 and the liquid lens 36is better, e.g., faster, than the thermal coupling between the liquidlens 36 and the temperature sensor 74. This is at least partially due tothe physical contact with the liquid lens 36 and the actuator 96.

Accordingly, the undesirable actuator induced temperature variations inthe liquid lens 36 can be controlled by controlling a bias signal 66 tothe actuator 96. The bias signal 66 can be applied when the controlsignal 64 is not being applied to the actuator for adjustment of thefocus of the lens for an image acquisition, thereby controlling theinduced temperature variations and the associated induced drift effects.The bias signal 66 through the actuator can be controlled to reduce thetemperature variations caused by internal heating and/or ambienttemperature.

Referring to FIG. 9, liquid lenses are commonly operated where theliquid lens is driven to return to a default position 100, typically inthe middle 102 of the focal range 104, after each focus operation 106.The default position 100 fails to consider any past operation of theliquid lens, e.g., if the liquid lens 36 was recently driven at a highfocal power or a low focal power. As seen in FIG. 9, the liquid lens 36was driven at a higher focal power more than it was driven at a lowerfocal power. This operation would typically increase the temperature ofthe liquid lens, thereby inducing drift effects and reducing thesharpness of acquired images.

Referring to FIG. 10, instead, in some embodiments, the bias signal 66to the actuator 96 can be controlled in such way that the average heatdissipation by the liquid lens 36 and actuator 96 is kept generallyconstant. Constant heat dissipation can equate to a constanttemperature, and a constant temperature can equate to a reduction or nodrift effects. For example, a history 68 of the liquid lens operationcan be maintained in memory 56, and the processor 50 can instruct areturn position based on an analysis of the past history. For example,if the liquid lens 36 was driven to the same focal powers as shown inFIG. 9, the processor can determine that the liquid lens 36 wouldincrease in temperature. Instead of returning the liquid lens 36 to themiddle of its focal range 102, the liquid lens 36 could be returned to adesired focal power position 110 with the bias signal 66, where the biassignal could be reduced enough to counterbalance the higher controlsignal 64 used for the higher focal powers. The processor 50 can managethe application of the bias signal 66 to the actuator 96 to average thecurrent applied to the actuator to reduce the induced temperaturevariations and the associated induced drift effects.

Similarly, the bias signal 66 to the actuator 96 can be controlled insuch a way that the bias signal is dependent on the measured temperatureof the liquid lens 36 to reduce the induced temperature variations andthe associated induced drift effects. For example, the liquid lens 36can be driven with a bias signal 66 that temporarily decreases after theliquid lens 36 has been set to a high optical power for an imageacquisition, and temporarily increases after the liquid lens has beenset to a low optical power.

Referring to FIG. 11, method 114 shows where a temperature factor 116 ismaintained and tracked for query by the processor 50. The temperaturefactor 116 can be a value associated with the amount of time a specificcontrol signal 64 is applied to the liquid lens 36. In this example, thetemperature factor 116 does not include a measured temperature value132, although in some embodiments, a measured temperature value 132 maybe included. When the liquid lens 36 is not being actively driven by thecontrol signal 64 for an image acquisition, the processor 50 can adjustthe bias signal 66 to compensate for the past control signal applied. Atprocess block 120, the processor 50 drives the liquid lens 36 for aspecific amount of time at a specific control signal 64 to acquire animage. At process block 122, a time value 112 for the specific amount oftime the specific control signal is applied and a control value 118 fora specific control current can both be stored in memory 56 as elementsof the temperature factor 116 (see FIG. 5). After the image has beenacquired and the temperature factor 116 has been stored, the processor50 can query the temperature factor 116 from memory, at process block124, in order to compute a return position for the liquid lens, based onthe temperature factor 116.

As a non-limiting example, if 100 milliamps of control signal 64 wasapplied to the actuator 96 for 10 milliseconds, the processor 50 canthen determine that the liquid lens 36 should be driven with a biassignal 66 current of 10 milliamps for 100 milliseconds to lower thetemperature of the liquid lens 36 to the control temperature 60. Atprocess block 126, the processor 50 can then drive the liquid lens tothe return position based on the analysis of the temperature factor 116.The method can repeat at process block 120.

Depending on when the liquid lens 36 is driven to a focal power duringuse of the reader 20, a counter 128 operable in memory 56 andcontrollable with the processor 50 can be included to count up or downto track the temperature factor. For example, the liquid lens 36 may bedriven to a new position prior to the completion of the application of10 milliamps for 100 milliseconds. The counter 128 can keep track of howmuch of the 10 milliamps for 100 milliseconds has been applied, andcontinue the application of the bias signal 66 after the liquid lens 36has completed the image acquisition. It is to be appreciated that theseare examples only, and many factors would affect specific bias signalsand application times, as would be understood by one skilled in the art.

Referring to method 130 in FIG. 12, in some embodiments, the temperaturesensor 74 can be read to provide a temperature value 132, and dependingon the temperature value 132, alone or in combination with thetemperature factor 116, the bias signal 66 can be controlled, i.e.,reduced or increased bias signal, in an effort to maintain a consistentand/or predetermined control temperature 60. Use of the temperaturesensor 74 has the benefit of including ambient or external temperaturesaffecting the reader 20, and specifically on the liquid lens 36. Atprocess block 134, a temperature value 132 is acquired from thetemperature sensor 74. Optionally, the temperature value 132 can bestored in memory 56 (see FIG. 5), at process block 136. After the imagehas been acquired and the temperature value 132 has been stored, theprocessor 50 can query the temperature value 132 from memory 56, atprocess block 138, in order to compute a return position for the liquidlens 36, based on the temperature value 132. At process block 140, theprocessor 50 can then drive the liquid lens 36 to the return positionusing a bias signal 66 based on the temperature value 132 and/or thetemperature factor 116. In addition, in some embodiments, tracking thetemperature factor 116 can be eliminated. The method can repeat atprocess block 134.

In some applications, the induced drift may not be able to be completelyeliminated, such as when the reader device is subject to large ambienttemperature swings, or the liquid lens 36 is operated in such a way thatthere is insufficient time to control the bias signal 66 to control thetemperature of the liquid lens, for example. In these applications, theimage sharpness can be determined over a series of images, alone or incombination with controlling the temperature of the liquid lens 36, toadjust a focal distance of the lens.

In most reader applications, a series of images is typically acquired.The series of images can be acquired either within one trigger, such asin a known continuous or manual mode, or over several triggers, such asin a known single trigger mode. An image acquisition parameter, e.g., afocal distance, can be changed by a predetermined small adjustment step142 between each of the series of images. For one or more of the imagesin the series of images, the reader 20 can use a sharpness calculation146 operable in memory 56 to determine a sharpness score 148 for eachimage. The sharpness score 148 from one image can be compared to asharpness score from another image to determine the effect of thepredetermined small adjustment step 142 between each of the images. Thepredetermined small adjustment step 142 can improve the sharpness score,or it can reduce the sharpness score, or the sharpness score can remainunchanged. Based on the comparison of the sharpness scores, theprocessor 50 can determine a direction, e.g., greater or less focaldistance, for a next predetermined small adjustment step. In someembodiments, alone or in combination with the sharpness score 148, theprocessor 50 may also use the ambient temperature change, e.g., anincrease or decrease in ambient temperature, to determine a direction ofthe predetermined small adjustment step 142.

Referring to FIG. 13, in some embodiments, the sharpness calculation 146can analyze a small region of interest (ROI) 152 within the field ofview of one or more images. At process block 154 of method 156, the ROI152 can either be defined automatically by a symbol, e.g., the barcode22 as seen in FIG. 1, or the ROI can be defined by the user, e.g., thehashtag symbol 160 as seen in FIG. 1. For example, the sharpnesscalculation 146 process can be enabled by placing a known ROI 152, e.g.,barcode 22 or symbol 160, within the field of view 42 for each imagewhere a sharpness score 148 is going to be calculated. The focaldistance of the adjustable lens 36 can be adjusted by the predeterminedsmall adjustment step 142 at process block 158. At process block 162, animage can be acquired that includes the ROI 152. Optionally, theprocessor 50 can confirm the ROI 152 is in the acquired image, atprocess block 163. At process block 164, the processor 50 can then runthe sharpness calculation 146 on the known ROI 152 identified in theimages to generate a sharpness score 148 for the ROI 152 in the acquiredimage. Next, at process block 166, the focal distance of the adjustablelens 36 can again be adjusted by the predetermined small adjustment step142. At process block 168, an additional image of the field of view thatincludes the ROI 152 can be acquired. Again, optionally, the processor50 can confirm the ROI 152 is in the acquired image. At process block170, the processor 50 can then run the sharpness calculation 146 on theknown ROI 152 identified in the additional image to generate asubsequent sharpness score 148. The first sharpness score 148 can becompared to the subsequent sharpness score 148, at process block 172.Based on the comparison of the sharpness scores, at process block 174,the processor 50 can define a direction for the next predeterminedadjustment step, and the focal distance of the adjustable lens 36 can beadjusted in the defined direction by the predetermined small adjustmentstep 142. The method can then repeat at process block 168 by acquiringanother image including the ROI 152 and comparing the sharpness scorewith the previously calculated sharpness score.

To make sure that the reader 20 doesn't slowly focus away from thepotentially small ROI 152 to the background due to drift, thepredetermined small adjustment step to the focal distance can belimited. This can include limiting adjustments to one image acquisitionparameter at a time, and/or limiting an amount of an adjustment to theone or more of the image acquisition parameters.

Although the present technology has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the technology. For example, the present technology is notlimited to reducing temperature induced drift effects on a liquid lensused in a machine vision system, and may be practiced with other systemsincorporating liquid lenses. For example, although a fixed-mount systemis shown and described above, the machine vision system can be ahand-held system. In a hand-held system, the distance from the visionsystem to a symbol or character to be read can be known or determined,and under these circumstances, adjustment of the focus can, in someapplications, be simplified.

The particular embodiments disclosed above are illustrative only, as thetechnology may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the technology.Accordingly, the protection sought herein is as set forth in the claimsbelow.

What is claimed is:
 1. A vision system comprising: an adjustable focusliquid lens having a field of view, a focus of the liquid lens beingadjustable with a control signal applied to the liquid lens via aconductor for capture of an image, the focus being adjustable by varyinga magnitude of a voltage or a current of the control signal; a biassignal applied to the liquid lens via the conductor at a time when theliquid lens is not being adjusted with the control signal for capture ofthe image; and the bias signal applied to the liquid lens to maintain atemperature of the liquid lens within several degrees of a particularoperating temperature.
 2. The vision system of claim 1, furtherincluding a processor operable to store a temperature factor in amemory.
 3. The vision system of claim 2, wherein the processor isoperable to analyze the temperature factor to generate the bias signalwith a magnitude configured to maintain the temperature of the liquidlens within several degrees of the particular operating temperature. 4.The vision system of claim 2, wherein the temperature factor includes atime value for a specific amount of time the control signal is appliedto the liquid lens, and the processor is operable to generate the biassignal based on the specific amount of time the control signal wasapplied.
 5. The vision system of claim 2, wherein the temperature factorincludes a control value for a specific current applied to the liquidlens, and the processor is operable to generate the bias signal based onthe control value.
 6. The vision system of claim 1, wherein the controlsignal comprises at least one of a control voltage and a controlcurrent.
 7. The vision system of claim 1, further including atemperature sensor to measure a temperature value of the liquid lens. 8.The vision system of claim 7, further including a processor operable toanalyze the temperature value to generate the bias signal to drive theliquid lens to the return position.
 9. The vision system of claim 7,wherein the temperature value is stored in a memory.
 10. The visionsystem of claim 1, wherein the bias signal is based on a history of thecontrol signal applied to the liquid lens.
 11. A method for reducingtemperature drift effects in a liquid lens, the method comprising:applying, via a conductor, a control signal having a first magnitude tothe liquid lens having a field of view and a focus at a first time,wherein the control signal adjusts the focus of the liquid lens forcapturing an image, the focus being adjustable by varying a magnitude ofa voltage or a current of the control signal; determining, based on thefirst magnitude, a second magnitude at which to apply a bias signal tomaintain a temperature of the liquid lens within several degrees of aparticular operating temperature; applying, via the conductor, the biassignal having the second magnitude to the liquid lens at a second timebetween image captures, the focus being adjustable by varying amagnitude of a voltage or a current of the bias signal, wherein the biassignal controls a temperature of the liquid lens and directly adjuststhe focus of the liquid lens; and applying the control signal having athird magnitude to the liquid lens at a third time subsequent to thefirst time and the second time, wherein the third magnitude is differentthan the first magnitude and the second magnitude.
 12. The method ofclaim 11, further including storing a temperature factor in a memory.13. The method of claim 12, further including analyzing the temperaturefactor to determine the second magnitude at which to apply the biassignal to maintain the temperature of the liquid lens within severaldegrees of the particular operating temperature.
 14. The method of claim12, wherein the temperature factor includes a time value for a specificamount of time the control signal is applied to the liquid lens, themethod further comprising determining the second magnitude at which toapply the bias signal based on the specific amount of time the controlsignal was applied.
 15. The method of claim 12, wherein the temperaturefactor includes a control value for a specific current applied to theliquid lens, the method further comprising determining the secondmagnitude at which to apply the bias signal based on the control value.16. The method of claim 11, wherein the control signal comprises atleast one of a control voltage and a control current.
 17. The method ofclaim 11, further including measuring a temperature value of the liquidlens.
 18. The method of claim 17, further including storing thetemperature value in a memory.
 19. The method of claim 17, furtherincluding analyzing the temperature value to generate the bias signal tocontrol the temperature of the liquid lens.
 20. The method of claim 11,wherein the bias signal is based on a history of the control signalapplied to the liquid lens.